Structure and Function of the Gastrointestinal Tract in Dogs
Dr. Bari Spielman
Mouth. Food is initially grasped by the teeth and tongue and enters the mouth. As the food is swallowed, it passes into the back of the mouth, which is known as the pharynx. Both food and air pass through the pharynx on their way into the body.
What Is the Gastrointestinal Tract?
The gastrointestinal (GI) tract or system is responsible for processing and extracting nutrients from food and collecting and passing waste material from the body. It is a very long and winding tube, beginning in the mouth and ending at the anus, through which food is swallowed and collected, then broken apart and digested. It is also where the nutrients from food are absorbed into the body. The GI tract includes the mouth, teeth, tongue, pharynx, esophagus, stomach, small intestine and large intestine.
Where Is the Gastrointestinal Tract Located?
The GI tract is a large system that travels the length of the body. It starts at the mouth, extends into the throat, through the chest and abdominal cavities, and ends at the anus.
The esophagus is the connecting tube between the pharynx and the stomach. As food leaves the pharynx it enters the esophagus and travels down the neck and through the chest. The esophagus passes through the diaphragm (the muscle that separates the chest from abdominal cavity) and ends at the stomach.
The stomach lies in the front of the abdominal cavity, just behind the liver. It is situated between the esophagus and the small intestine, lying predominantly on the left side of the body.
The small intestine is located within the abdominal cavity and extends from the stomach to the junction of the small and large intestine.
The cecum is a small dead-end pouch that lies near the junction of the small and large intestines. The colon begins in the lower portion of the right side of the abdomen and travels forward along the right side, then crosses the midline, and proceeds back down the left side. This last portion of the colon (descending colon) leads into the rectum and then empties through the anus. The rectum is the terminal portion of the large intestine that passes through the pelvis and leads to anus.
What Is the General Structure of the Gastrointestinal Tract?
For most of its length, the GI tract is a long hollow tube lined by different types of cells. The walls of the tube are composed of glands, nerves and muscles. Structurally, the cell type, muscle thickness, glandular elements, and nervous supply differ in the various functional regions, as does the diameter and shape of the tube.
The esophagus is a rather straight tube that is lined with muscles that force food down the neck and through the chest towards the stomach. In a medium-sized dog it is about 15 to 18 inches long and an inch in diameter when collapsed. It is divided into cervical (neck), thoracic (chest), and abdominal portions.
The stomach is a large sac-like dilatation of the GI track and is made up of several distinct areas. The inlet or opening from the esophagus into the stomach is called the cardia. The exit or outlet of the stomach that leads to the small intestine is the pylorus. The stomach is shaped somewhat like a large lopsided kidney bean that lies across the front of the abdomen. The left side of the stomach, closest to the cardia is larger than the right side and is called the fundus. The smaller right-sided portion of the stomach that ends at the pylorus is called the body of the stomach. The lining of the stomach contains glands that produce acids and enzymes that digest food and the walls of the stomach contain muscles that mix and move the food. The glands of the stomach also produce mucous, which protects the stomach from being digested by its own acid and enzymes.
The small intestine is the longest portion of the GI tract. It is a circular hollow tube that is approximately three to four times the length of the animal's body. The interior lining of the small intestines has numerous microscopic, finger-like projections called villi. These villi stick out towards the center of the intestine and greatly increase the surface area available for digestion and absorption.
The small intestine consists of the duodenum, the jejunum and the ileum. The duodenum is the first and most stationary part of the small intestine. Within the duodenum, openings are present that allow digestive juices to enter the intestines from the pancreas and gallbladder. The jejunum is the longest part of the small intestine and is free to move into whatever unoccupied space is available within the abdomen. The ileum is the short, terminal portion of the small intestine.
The large intestine is wider and shorter than the small intestine. It includes the cecum, colon, rectum, and anal canal. The cecum is a comma-shaped pouch that lies at the junction of the ileum and colon. The colon is shaped like a question mark. It is thin-walled and baggier than the small intestines. The rectum is the last few inches of the colon and leads directly to the anal canal. The anal canal is the short, terminal part of the GI tract that lies just inside the anus. It is only about one-half inch in length. The anus has two muscular sphincters that act as a door, holding the stool (fecal material) inside the body until it is appropriate to defecate.
The components of the GI tract that lie within the abdomen are held in place by their attachments to the mesentery. The mesentery is a curtain-like structure that hangs from the top of the middle of the abdomen. It contains blood vessels that travel to and from the GI tract. It also contains lymph vessels that carry certain nutrients away from the GI tract.
What Are the Functions of the Gastrointestinal Tract?
The esophagus acts like a conduit, a tube that moves ingested material from the mouth to the stomach. Wavelike contractions (called peristalsis) move food from the mouth down the neck, through the chest and into the stomach. The esophagus has a tight sphincter muscle where it meets the stomach. This lower esophageal sphincter is a ring of thick muscle that acts as a door, and it prevents acid reflux or movement of acid from the stomach back up into the esophagus.
The stomach has three basic functions that assist in the early stages of digestion and prepare the food for further processing in the small intestine. First, it serves as a short-term storage area, allowing the animal to consume a large meal quickly and processing it over a longer period of time. Second, substantial chemical and enzymatic digestion begins in the stomach, particularly of proteins. Third, the stomach's contractions mix and grind food with secretions, liquefying or blending the food, a necessary step before the food is delivered to the small intestine.
The small intestine is where the absorption of almost all nutrients into the blood occurs. When in the small intestine, food particles are exposed to enzymes and bile, which convert the food to even smaller particles capable of being absorbed into the blood. In addition to absorbing food particles, the small intestines also the absorb other materials such as water, electrolytes and other molecules. The small intestines provide nutrients to the body and play an important role in water and acid-base balance.
The large intestine participates in the last phase of digestion. It has three very important functions. It recovers the last available water and electrolytes from the food,; forms and stores feces, and works with bacteria to produce enzymes capable of breaking down difficult-to-digest material.
The rectum and anal canal are basically collecting spaces, where the feces are stored until it is appropriate for them to be defecated.